The production of ice in aqueous liquids has many useful purposes. Ice can be produced and used immediately, or the ice can be stored and used later, for example, for cooling purposes. Also, in the production of potable water, sea water and brackish water can be cooled to produce ice, the ice separated and then melted to give the desired fresh water. Fruit and vegetable juices are also concentrated by cooling them to produce ice and then separating the ice from the concentrated juice.
The production of ice for the described purposes, as well as others, can be achieved in a number of ways including indirect heat transfer in a shell and tube freeze exchanger. A refrigerant can be used as the cooling medium on the shell side of the freeze exchanger. This method is disclosed in U.S. Pat. No. 4,286,436.
Another method of producing ice is to directly contact the aqueous liquid with a refrigerant. Direct contact heat transfer typically permits the use of a smaller temperature difference between the vaporizing refrigerant and freezing solution than is required by an indirect heat transfer system to achieve the same energy transfer, due to the elimination of the heat exchanger surface. However, the exact temperature difference required in the direct contact heat exchanger will depend upon several factors including the properties of the two fluids, the ratio of the two fluids and agitation. This method, as well as apparatus useful therefore, is disclosed in U.S. Pat. Nos. 3,017,751; 3,017,752; 3,259,181; 3,835,658; 3,885,399 and 4,046,534. After the ice is produced it is separated and then discarded, melted and used as potable water, or melted to recover stored refrigeration. The refrigerant used for cooling and ice formation is recovered to the extent possible and then reused in the process.
Experience has shown that in direct contact methods a significant amount of refrigerant vapor and liquid can be encapsulated or entrapped in the aqueous solution, either by gross inclusion in the ice crystals or by the formation of clathrate hydrates. When the ice or hydrate is later melted or disposed of, some or most of the encapsulated refrigerant may be difficult to recover. Additionally, encapsulation of the refrigerant constitutes a economic penalty because the amount of refrigerant required for continuous operation is substantially increased. Accordingly, it is desirable in the production of ice, by directly contacting an aqueous liquid with a refrigerant, if refrigerant capsulation in the water or ice could be reduced. Thus, only select chemical refrigerants are desirably used in direct contact heat transfer aqueous processes.
Many chemical refrigerants form clathrate hydrates with water at temperatures substantially above 32.0.degree. F. (0.0.degree. C). For example, chlorodifluoroethane (CH.sub.3 CClF.sub.2), commonly designated HCFC-142b, and 1,1-difluoroethane (CHF.sub.2 CH.sub.3), commonly designated HFC-152a, form hydrates with water at 56.degree. F. (13.1.degree. C.) and 60.degree. F. (15.3.degree. C.) respectively per Briggs, F.A. and Barduhn, A.J. "Properties of the Hydrates of Fluorocarbons 142b and 12B1", Advances in Chemistry Series, American Chemical Society, No. 38, pp. 190-199, 1963. Also, 1,1,1,2-tetrafluoroethane (CH.sub.2 FCF.sub.3), commonly designated HFC-134a, forms a hydrate with water at 50.degree. F. (10.degree. C.) per Mori, Yasuhiko H., and Mori, Tatsushi, "Formation of Gas Hydrate with CFC Alternative R-134a", AlChE Journal, Vol. 35, No. 7, July 1989, pp.1227-1228. Other chemical refrigerants which form hydrates are bromodifluoromethane (CHBrF.sub.2 ; HBFC-22B1) (50.0.degree. F.; 9.9.degree. C.); bromochlorodifluoromethane (CBrClF.sub.2 ; BCFC-12B1) (50.0.degree. F.; 9.9.degree. C.); methylene fluoride (CH.sub.2 F.sub.2 ; HFC-32) (63.7.degree. F.; 17.6.degree. C.); chlorofluoromethane (CH.sub.2 ClF; HCFC-31) (64.1.degree. F.; 17.8.degree. C.); chlorodifluoromethane (CHClF.sub.2 ; HCFC-22) (61.3.degree. F.; 16.3.degree. C.); dichlorofluoromethane (CHCl.sub.2 F; HCFC-21) (47.5.degree. F.; 8.6.degree. C.); dichlorodifluoromethane (CCl.sub.2 F.sub.2 ; CFC-12) (53.8.degree. F.; 12.1.degree. C.); trichlorofluoromethane (CCl.sub.3 F; CFC-11) (47.3.degree. F.; 8.5.degree. C.); cyclopropane (CH.sub.2 CH.sub.2 CH.sub.2 ; C-270) (63.degree. F.; 17.2.degree. C.); propylene (CH.sub.2 =CHCH.sub.3 ; C-1270) (33.7.degree. F.; 0.94.degree. C.); and methyl chloride (CH.sub.3 Cl; R-40) (70.0.degree. F.; 21.1.degree. C.), per Briggs and Barduhn (above) and Water: A Comprehensive Treatise, Volume 2, edited by F. Franks (Plenum Press, New York, 1973).
It is often advantageous if the chemical refrigerant used in a direct contact heat transfer process has a normal boiling point less than 32.0.degree. F. (0.0.degree. C.). In a process such as that described by Knodel et al in U.S. Pat. No. 4,596,120 the ice is accumulated in a vessel which operates essentially at the refrigerant evaporating pressure. If the refrigerant has a normal boiling point less than 32.0.degree. F. (0.0.degree. C.), than the vessel operates above atmospheric pressure, and the potential of air in leakage is minimized. This is of particular importance since large commercial systems typically require a number of field assemblies and joints.
Additionally, it is considered advantageous if chemical refrigerants do not contain the chemical element chlorine, as recent world environmental agreements have restricted the production and use of certain chlorine-bearing compounds, in an effort to protect the earth's stratospheric ozone layer.
Knodel et al U.S. Pat. No. 4,754,610 discloses refrigerants useful in that process and which do not form hydrates or react with an aqueous body. Those refrigerants include butane (R-600), octafluorocyclobutane (C-318), 1,2-dichlorotetrafluoroethane (CFC-114), and a mixture of CFC-114 and dichlorodifluoromethane (CFC-12) where less than 40% by weight of the mixture is CFC-12. Some of these listed refrigerants are flammable and some are considered to be environmentally harmful, and so their use will be prohibited in some countries before long. Apart from these disadvantages it would be beneficial in the art of ice making by direct contact of a liquefied refrigerant with an aqueous liquid if one could be able to select an alternative or substitute refrigerant from a group which it was known did not form clathrate hydrates to thereby design an optimum process for the conditions otherwise involved in the process.